Precision guidance for missiles is a subject of high interest for all military organizations throughout the world. The required precision has high costs due, in great part, to the complexity of the guidance techniques generally used. Radar, RF, GPS, TV, IR or lasers are examples of technologies that have been used to meet the guidance precision requirements. The majority of the present approaches are based on terminal homing seekers. The problem with these approaches is the high costs associated with seeker components such as gimbals, domes, high performance electronics and software. Other techniques make use of a human in the loop to reduce the complexity and costs of the components installed aboard a missile. In these techniques, a human operator provides assistance to a missile's guidance system by correcting its trajectory through a data link such that a satisfactory trajectory of the missile to the target is achieved. This approach substantially reduces the costs of the guidance system but the precision is directly proportional to the operator's skill and this can be highly variable.
Existing guidance systems for missiles may use a laser designator to provide guidance signals to the missile and these generally employ either a beam riding method or a semi-active terminal seeking method. In a semi-active terminal seeker guidance method, a sensor on board the missile detects laser energy that is reflected from a target being designated by a laser designator. Signals from the sensor are processed and outputs missile-to-target line-of-sight angles or angle rates to a guidance processor in the missile that calculates guidance commands to guide the missile to a target. In beam rider guidance systems, a missile flies inside a laser beam or beside laser beams so that guidance information can be determined from a laser scan pattern determined by one or more rearward looking detectors mounted on the missile. The scan pattern information is used by a processor on board the missile to calculate the guidance commands necessary to guide the missile to the center of the beam or maintain a predetermined distance from a beam's axis. Various techniques based on light scattered from laser beams have been used to provide guidance to missile during flight.
One technique using an operator to guide missiles during flight is described in U.S. Pat. No. 4,234,141 by Walter E. Miller, Jr. which was issued on 18 Nov. 1980. In this guidance system, an observer establishes and maintains a visual line-of-sight contact with a target through a telescope. In tracking the missile's trajectory towards the target, short pulses of collimated light are transmitted from the launch site towards the missile, which pulses are received by an optical receiver on the missile and are simultaneously reflected by a retro reflective prism on the missile. The reflected pulses follow a path parallel to the incident wave and are thus directed back to the launch site. A missile tracker at the launch site responds to the reflected pulses and measures any deviation of the missile from the visual line-of-site axis maintained between the launch site and the target. Guidance commands are then transmitted towards the missile for maintaining the missile on the proper trajectory, which pulses contain correctional signals for the missile's guidance system. The guidance precision of this technique is proportional to the observer's (operator's) skill in maintaining the visual line-of-sight contact with the target and this can be rather variable.
Another missile guidance system is a scatterider guidance system that utilizes a designating beam of laser pulses from the launch pad towards the target. Atmospheric particles scatter the laser pulses and sensors on the missile detect the laser light scattered by the atmospheric particles and onboard guidance electronics calculate the radial distance between the laser beam and the longitudinal axis of the missile. That distance is then used to generate deflection commands for the aerodynamic control surfaces of the missile such that it approaches and stays close to the laser beam. In the embodiment described by McCowan et al in U.S. Pat. No. 6,138,944, laser light scattered from atmospheric particles for one laser pulse is detected by eight aft-looking optical sensors mounted around the circumference of the missile and one annular forward-looking sensor mounted on the nose section. The aft-looking sensors are mounted with their centrelines angled such that they point 60 degrees aft of the perpendicular to the missiles' longitudinal axis and each has a field-of-view of 45 degrees so the eight sensors provide a 360 degree coverage around the missile. The detection of back-scatter laser light is provided by one annular forward-looking sensor that is mounted such that it stares at a 45 degree angle forward of the perpendicular to the missile's longitudinal axis.
In U.S. Pat. No. 6,138,944, when light scattered from a single laser pulse is detected by one of the eight aft-looking sensor and by the forward-looking sensor, this detection as well as the time lapse between the detection by the aft-looking sensor and detection by the forward-looking sensor are provided as inputs to guidance electronics. That time lapse will be directly dependent on the radial distance between the missile's longitudinal axis and the laser beam due to the time it takes the laser beam to travel from the aft-looking detection point and the forward-looking detection point. The guidance electronics is coupled to the aft-looking sensors to derive which one actually detected the scattered light and then calculates the radial distance between the laser beam and the missile using principles of geometry and trigonometry. That distance as well as the aft-looking detector that detected the scattered laser light is used to generate commands for the guidance system to maintain the missile on the desired trajectory with respect to the laser beam. The precision of this guidance system is dependent on the precision of the geometric and trigonometry calculations. The annular forward-looking sensor will limit what type of countermeasure may be effective since no sensor is looking directly at the target. It does, however, require inertial rate sensors and a roll sensor.
Another laser guidance system for a missile is described in JP Patent No. 2000039296 (Application No. JP 98209942) based on three laser beams directed along vertices of an equilateral triangle between which a missile flies and then determines its position by the relative level of laser scattered signal detected by a number of sensors on the missile.
A further laser guidance system for a missile is described by Walter E. Miller, Jr., in US Statutory Invention Registration H299 that was published on 7 Jul. 1987. In this system the operator maintains sight of a target through optics at a fire control station. A laser transmitter on the missile directs a laser beam towards the fire control station where it is received and coupled to a phase conjugated amplifier and redirected back through a spatial encoder to the missile. The encoding provides guidance instructions for the missile where the redirected beam is received and applied to the missile's guidance control system. This guidance system does not have any sensor directed toward the target and this limits what type of countermeasures may be effective. It does, however, require an operator in the loop that has to maintain visual sight of the target.
The majority of guidance techniques, other than those taught by McCowan et al and Walter E. Miller, Jr., share a common weakness in that they are susceptible to detection by the target which can employ countermeasure since the field-of-view of their guidance sensor have to continuously look at the target. Countermeasure that may be employed including dazzling or destruction of the sensor which would ruin the precision guidance of the missile. The majority of these missiles still rely on complex and costly gyroscopes and accelerometers to assist in guidance.
A new generation of hypervelocity missiles presently being developed inevitably call for a highly profiled fuselage nose that denies any possibility of using a forward-looking sensor because that would require a dome at the tip of the missile. There is, as a result, a requirement for a new guidance system for missiles that would be almost immune to known countermeasures while permitting use of a highly profiled nose and provide for a low cost implementation.